I will also add that citrus peels can also be used to make essential oils, which can be very valuable. Either press the peels or steam distillation, the latter of which can be helpful to avoid photosensitizing furanocoumarins from getting in the oil (this can be an issue with bergamot, lime, lemon, and to a much lesser extent grapefruits/pomelo).
Satsuma mandarin oil and yuzu oil are especially sought after, while pomelo has a very perfume-like aspect but the essential oil is pretty uncommon to be able to find.
d-limonene is the major component of orange oil and is a very useful safe natural solvent.